Earth Planets Space, Vol. 65 (No. 6), pp. 551-562, 2013
Haruhisa Nakamichi, Yoshiko Yamanaka, Toshiko Terakawa, Shinichiro Horikawa, Takashi Okuda, and Fumihito Yamazaki
Earthquake and Volcano Research Center, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
(Received October 30, 2012; Revised January 16, 2013; Accepted March 4, 2013; Online published July 8, 2013)
We deployed a seismic array at a site 5 km east of
Shinmoedake volcano, in the Kirishima volcanic complex of southwest
Japan, five days after the sub-Plinian eruption on 26 January, 2011.
The array record between February and September 2011 included
explosion earthquakes and episodes of weak continuous tremor during
eruption periods. We estimated slownesses and back azimuths of
seismic waves on a sliding 1-min window using the semblance method.
The slownesses of the weak continuous tremor clustered within the
range 0.2-0.8 s/km, consistent with a mix of body and surface
waves. A probabilistic approach based on a grid search was used to
estimate the source locations of the explosion earthquakes and weak
continuous tremor. The sources of the explosion earthquakes were
beneath the crater at depths of -0.5-1 km above sea level, while
the source of the weak continuous tremor was beneath the northern
part of Shinmoedake at depths between 1 km below sea level and 1 km
above sea level. This latter region corresponds to a shallow
low-resistivity layer, suggesting that hydrothermal processes are
more plausible than magmatic processes as the generating mechanism
of the weak continuous tremor.
Key words: Explosion earthquake, volcanic tremor, seismic array, semblance analysis.